Some pint-size birds, mammals have amazing strategies to cope with winter's cold

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It's a general rule of nature that, in winter, bigger is better. Other things being equal, large animals tend to have an advantage over smaller ones when temperatures plummet. Among species that occupy large ranges covering whole hemispheres, individuals at northern latitudes tend to be bigger than those in the south. Minnesota white-tailed deer, for example, attain greater weights and physical dimensions than deer from Texas. It's a simple fact of physics that a large mass retains heat more efficiently, so bigness can be a winning survival strategy.

But it's another rule of nature that there are no absolutes. Despite diminutive size, plenty of tiny creatures scratch out a living in the face of winter's challenge. All have devised behaviors or metabolic safeguards that get them through until spring.

Most small bird species -- our warblers are good examples -- simply fly away from winter's harshness. Their seasonal migrations to tropical haunts keep them in mild temperatures and ample food. But some small birds, many of which are familiar to winter bird-feeding enthusiasts, stay in cold climates year-round. These year-round residents employ some amazing adaptations to thrive in cold and snow.

Small non-migratory birds like chickadees, titmice and nuthatches face a physiological balancing act every cold winter night. Their small size means their ratio of surface area to volume is high, so they lose body heat quickly. Their only recourse is to eat a lot of calories to produce more heat. In that regard being small is an advantage; they can get by on less food than bigger birds.

"The thing that really impressed me about chickadees is their metabolism," Penn State professor of wildlife resources Margaret Brittingham stated in an article published in National Wildlife magazine about her winter studies of chickadees in Wisconsin. "We weighed birds in the morning and found that they had virtually no body fat. Yet the same birds examined in the afternoon of the same day were bulging with fat."

Chickadees also can self-induce a state of torpor; they slow down their own metabolism to burn fewer calories during a long winter night.

"Carefully hidden food items, dense winter coats, specially selected winter roost cavities, and perhaps most remarkable of all, the ability to go into nightly hypothermia, thus conserving large amounts of energy, greatly increase the chance of survival," stated Cornell ornithologist Susan M. Smith in the same article.

Smith's studies found that chickadees can drop their base body temperature by as much as 15 degrees below the 108-degree daytime norm, saving as much as 25 percent of their metabolic budget during nighttime hours.

Torpor has a risk, though. It impairs birds' ability to respond to predators or other sudden dangers.

Chickadees, titmice and small woodpeckers like the downy and hairy woodpeckers also scavenge fat from dead carcasses as a valuable calorie supplement to their normal diet of seeds and dormant insects. Troops of chickadees often scavenge fat from deer entrails left behind by hunters during the deer season.

Small birds can also "fluff" their feathers while roosting at night, creating air spaces between feathers to improve their insulating quality.

Chickadees also roost together in communal tree cavities on cold nights. Their combined heat loss can warm the cavity interior to above freezing, even when air temperatures outside are below zero.

A number of familiar small mammals have their own adaptations that favor winter survival. Some, like the woodchuck, jumping mouse and all 11 species of bats native to Western Pennsylvania are true hibernators. They pass the winter underground or in caves in a long state of metabolic arrest.

Most mammals, though, tough the winter out in other ways. An adaptation that helps the chipmunk survive winter is its internal cheek pouches, where it temporarily holds large volumes of food during transport to a cache inside the winter den. The cheek pouches can hold a surprising volume of edible material, thus allowing the chipmunk to cache more food while minimizing its exposure to weather and predators. According to a Guide to the Mammals of Pennsylvania (University of Pittsburgh Press), one biologist counted 32 beechnuts inside the pouches of one chipmunk. Another researcher in Vermont counted 60 sunflower seeds in the cheek pouches of a road-killed chipmunk. The den itself can hold a half-bushel of hoarded nuts, seeds and other plant material.

Like the chickadee, the chipmunk endures severe cold by entering self-induced torpor. The chipmunk doesn't truly hibernate but can slow down its metabolism to conserve energy for as long as eight or 10 days while it sleeps fitfully inside its den.

The red-backed vole, a common small mammal of Pennsylvania's forested mountain regions, doesn't hibernate but constructs elaborate tunnel systems to reach food sources buried beneath snow. If snow is deep enough, temperatures inside these tunnels can remain around freezing while the air above is much colder. If snow melts off during winter voles can retreat to similar, though more restricted, tunnel systems under leaf litter and among tree roots.

The familiar white-footed mouse stays active all winter but it gets through cold spells by communal nesting. Several mice huddle together in tight burrows in hollow trees or underground, warming the air within the den.

Ben Moyer is a freelance writer living in Farmington, Pa.

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